What Is a Rain Garden?
A rain garden is a shallow bowl-shaped dip in the landscape that collects rainwater. They are often planted with native plants and can be designed with a formal or informal aesthetic. A rain garden is a great place to direct rain runoff from roofs or paved areas, as well as the overflow from another rainwater collection system such as a rain barrel. They can even be used to help manage or drain a naturally wet area in your yard. Rain gardens add beauty to your landscape while managing the rain and providing habitat for birds, bees, and other pollinators.
How Rain Gardens Work
In general, rain from a roof or paved area is directed to the rain garden where it is collected and stored until it can safely soak into the ground. The rain garden’s plants and soil filter out chemicals, dirt, and other pollutants picked up by the rainwater as it washes over hard surfaces. This keeps rainwater out of the sewer system, helps reduce the risk of sewer backups or overflows to the Willamette River, protects our rivers and streams from pollution, and replenishes groundwater.
Summary of Rain Garden Design Requirements
These site and design requirements can help you decide if a rain garden might be appropriate for your project.
Rain gardens are not very deep, so they can be created without professional help if you pay attention to important safety measures.
To stay safe and avoid damaging buildings or other structures:
- The edge of the garden must be:
- At least 6 feet away from a building with a basement or 2 feet from a building without a basement.
- At least 5 feet away from neighboring properties.
- At least 5 feet from the base and 10 feet from the top of retaining walls higher than 3 feet.
- The deepest part of the rain garden should be at least 10 feet from any neighboring structures.
When building a rain garden, it’s also important to keep in mind:
- The infiltration rates of the soils (how well water soaks into the ground). An infiltration test is recommended prior to rain garden construction. Find information on how to do an infiltration test in the How to Build a Residential Rain Garden guide below on this page.
- The minimum suggested ponding depth of the finished rain garden should be 6 to 12 inches. Rain gardens should completely drain within 24 hours of a rain event.
- Be cautious of underground utilities. Do not build over water, gas lines or oil tanks. Call before you dig, 1-800-332-2344, 8-1-1, or schedule an appointment online to locate all underground utilities.
- Rain gardens are not suitable for steep locations — property with more than a 10 percent slope.
- Avoid compacting the native soils. The rain garden must be large enough to handle the runoff directed to it. Sizing will depend on tested infiltration rates and catchment area.
- Every garden should have a safe escape route. Plan where the rain will go when the garden is full and direct it away from structures and neighboring properties.
When choosing plants for your rain garden:
- Install plants from the Stormwater Management Manual plant list or choose plants appropriate for the native plant community type as described in the Portland Plant List.
- Environmental Services prohibits plants on the Portland Nuisance Plant List and the Required Eradication List. Both categories can be found in the Portland Plant List. Find a link in the Resources section below.
When to Call a Professional
A professional designer is not required to design and build most home rain gardens. However, if you want help in selecting your plant palette, a nursery professional can help you pick out suitable plants based on your soil, sunlight, and garden design.
Costs and Permits
Rain gardens are not approved for any new construction or redevelopment project that activates the Stormwater Management Manual requirements. See the manual for more information.
The cost of installing a rain garden depends on many factors such as size, plant selection and density, and other possible work such as removing paved surfaces or rerouting downspouts.
You will need a City permit if your project involves any of the following activities or if any of the following conditions apply to your property.
- You excavate or remove more than 10 cubic yards of dirt (that’s about enough to fill one standard size dump truck).
- Your property has a 10 percent slope or more.
- Your property is within 50 feet of a wetland or waterbody. Your property is in a floodplain.
- You do not need city permits to construct a residential rain garden if:
If any of these conditions apply to your property, you may need to take extra steps to safely install a rain garden. Contact the Private Property Drainage Inquiries team to discuss safe options for your property.
Like any garden, a rain garden requires some regular maintenance. Once the garden becomes established, which happens in about two years, maintenance should be minimal. Because they collect lots of water, it is important to inspect your rain garden periodically, especially after a heavy rain.
Other ongoing maintenance tasks include:
- Water the plants deeply once a week during dry months (May to October) to encourage root growth and keep plants strong, especially while plants are getting established during the first two summers.
- Pull weeds by hand before they become established (avoid chemical weed killers).
- Remove sediment and debris, watch for erosion, and replace plants as needed.
- Regularly check gutters, downspouts and inlet pipes to ensure they are free and clear of debris and that rainwater can enter the garden.
- Once a year, layer compost or mulch 2 inches deep to suppress weeds and feed plants.
- Thin and prune plants as needed. Divide dense plantings every two to five years.
- Remove leaves in the fall. Leaf build-up will reduce rain garden capacity and smother plantings.
Stormwater Management Manual
Consult the City’s Stormwater Management Manual for the complete set of requirements on how to safely site, build, and maintain a rain garden or other stormwater management solution on your property.
Clean River Rewards Eligibility
Residential and commercial property owners who install qualified stormwater management solutions may be eligible for a discount on the stormwater charges of their sewer, stormwater, and water bill through the Clean River Rewards program.
Rain gardens that meet the safety and space requirements outlined above and are at least 10 percent of the area of the roof that is draining to it are eligible for Clean River Rewards. Visit Clean River Rewards to learn more.
Find More Resources
How to Build a Residential Rain Garden. This printable how-to guide from Environmental Services will walk you through the steps to plan, design, and build your rain garden.
Low-Impact Development Fact Sheet on Rain Gardens by Oregon State University Extension Service provides a detailed overview of all aspects of rain gardens from site conditions to maintenance. Get the Low Impact Development Fact Sheet
Free Classes. East Multnomah Soil and Water Conservation District offers free classes on building rain gardens. Learn more or sign up for Rain Gardens 101.
Stormwater Management Manual's Plant List provides a list of plants recommended for rain gardens.
The Portland Plant List contains lists of native and nuisance plants within Portland.